my way with my thoughts: 2020

MECHANISM OF FLAGELLAR MOTILITY,MOTILITY DUE TO OTHER STRUCTURE

INTRODUCTION-

- The bacterial flagellum is a helical filamentous organelle responsible for motility.

- Flagella present at cell exterior, the long helical flagellar filament acts as a molecular screw to generate thrust.

- Flagellated bacteria share a common rotary nanomachine known as flagellar motor which is located at the base of the filament.

- The flagellar motor is composed of a rotor ring complex and multiple transmembrane stator units and converts the ion flux through an ion channel of each stator unit into the mechanical work required for motor rotation.

- Motility can be a crucial virulence attribute for pathogenic bacteria, such as Salmonella enteric.

- Bacterial motility also plays a significant role in mutualistic symbioses .

- Active motilities of bacteria are represented by movement in liquid and on solid surface and passive motility is typically actin-based locomotion .

- Since bacterial motility varies among bacterial species, bacteria utilize their own motility system optimized for their habitats.

- E. coli and Salmonella use flagella viewable from the cell exterior as a thin, long, helical filament.

- On the other hand, the flagella of spirochetes reside within the periplasmic space so they are called periplasmic flagella .

- The flagellum is divided into three structural parts:

- the basal body as a rotary motor

- the hook as a universal joint

- and the filament as a molecular screw in common

- and flagellar formation and function involves more than 60 genes.

- The bacterial flagellar motor is powered by the transmembrane electrochemical gradient of ions namely ion motive force (IMF) and rotates the flagellar filament to generate thrust to propel the cell body.

- The flagellar motor is composed of a rotor and multiple stator units.

- Each stator unit acts as a transmembrane ion channel to conduct cations such as protons (H⁺) or sodium ions (Na⁺) and applies force on the rotor .

- The flagellar motors of E. coli and Salmonella rotate in both counterclockwise (CCW) and clockwise (CW) .

AXIAL STRUCTURE-

- The axial structure of the bacterial flagellum is commonly a helical assembly composed of 11 - protofilaments.

- It is divided into at least three structural parts:

- the rod,

- the hook

- and the filament from the proximal to the distal end.

- The rod is straight and rigid against bending and twisting and acts as a drive shaft.

- The hook is supercoiled and flexible against bending and acts as a universal joint to smoothly transmit torque produced by the motor to the filament.

- The filament is also supercoiled but stiff against bending.

- The filament is normally a left-handed supercoil to act as a helical screw to produce thrust for swimming motility.

- The filament undergoes polymorphic transformation from the left-handed supercoil to right-handed ones when bacterial cells tumble and change swimming direction .

FLAGELLA FILAMENT-

- The flagellar filament of E. coli is formed by ~30,000 copies of flagellin, FliC.

- Salmonella has the fljB gene encoding another flagellin subunit in addition to the fliC gene.

- Because flagellin is a major target of host immune system such an additional flagellin subunit enables Salmonella cells to escape from adaptive immune response of the host more efficiently compared to E. coli cells .

- Salmonella FliC is composed of four domains

- D0, D1, D2 and D3

- Arranged from the inner to the outer part of the filament structure.

- Domains D0 and D1 are well conserved among bacterial species.

- Domains D2 and D3 are variable even among Salmonella spp because these two domains are the major targets of antibodies .

- The supercoiled forms of the filament structure are generated by combinations of two distinct left-handed (L-type) and right-handed (R-type) helical conformations of flagellin molecule and packing interactions of the L- and R-type protofilaments .

- so the helical properties of each supercoil are determined by a ratio of L-type protofilaments to R-type ones in the filament structure .

- The flagellar filament Because the assembly of the flagellar filament by multiple flagellins affects its mechanistic properties for flagellar function in different environments .

HOOK AND ROD -

The Salmonella hook is formed by about 120 copies of the hook protein FlgE.

Salmonella FlgE consists of three domains D0, D1, and D2, arranged from the inner to outer parts of the hook structure

The hook forms several supercoils and axial interactions between a triangular loop of domain D1 and domain D2 are responsible for hook supercoiling.

Basal Body Rings

- The basal body has multiple ring structures, namely L ring, P ring, MS ring, and C ring .

- The L and P rings, which are formed by the lipoprotein FlgH and the periplasmic protein FlgI, respectively are embedded in the outer membrane and the peptidoglycan (PG) layer.

- The LP ring complex is missing in the basal body of gram-positive bacteria such as B. subtilis. In contrast, the MS and C rings are well conserved among bacterial species .

- The MS ring is composed of the transmembrane protein FliF and is part of a rotor .

- FliG, FliM, and FliN form the C ring on the cytoplasmic face of the MS ring.

- The C ring acts not only as a central part of the rotor for torque generation .

Stator-

- The transmembrane stator unit of the flagellar motor conducts ions and exerts force on the rotor.

- Based on the coupling ion and sequence similarity, the stator units are classified into three groups:

- H⁺-coupled MotAB complex,

- Na⁺-coupled PomAB complex,

- and Na⁺-coupled MotPS complex .

- The MotAB complex is composed of four copies of MotA and two copies of MotB and acts as a transmembrane H⁺ channe.

- The PomAB and MotPS complexes form a Na⁺ channel in a way similar to the MotAB complex .

MECH-

- Each of the outer 9 doublet microtubules extends a pair of dynein arms to the adjacent microtubule.

- These produce force through ATP hydrolysis.

- The flagellar axoneme also contains radial spokes, polypeptide complexes extending from each of the outer nine microtubule doublets towards the central pair, with the "head" of the spoke facing inwards.

- The radial spoke is thought to be involved in the regulation of flagellar motion, although its exact function and method of action are not yet understood.

H+ TRANSLOCATION MECHANISM-

- The maximum rotation rate of the H⁺-driven flagellar motors of E. coli and Salmonella is reduced with a decrease in the intracellular pH.

- These observations suggest that the intracellular H⁺ concentration affects the rate of the H⁺ flow through the MotAB complex.

- The motB mutation results in a considerable decrease in the rate of H⁺-coupled conformational change of the MotAB complex .

- The motB(D33E) mutation causes not only large speed fluctuations but also frequent pausing of motor rotation at low load.

- However, neither speed fluctuation nor pausing is seen at high load .

- Based on MD simulation of the H⁺ channel of the E. coli MotAB complex, the H⁺ translocation through the channel is postulated to be mediated by water molecules aligned along a H⁺ pathway.

TORQUE GENERATION-

ROTATION MECHANISM-

- Highly conserved Arg-90 and Glu-98 residues of MotA, which are located in the cytoplasmic loop between TM2 and TM3 of MotA, interact with highly conserved Asp-289 and Arg-281 residues of FliG, respectively.

- These two electrostatic interactions are responsible for efficient stator assembly around the rotor, and the interaction between Glu-98 of MotA and Arg-281 of FliG is likely to be involved in torque generation.

- H⁺ translocation through the transmembrane H⁺ channel of the MotAB complex allows the cytoplasmic loop of MotA to associate with and dissociate from FliG to drive flagellar motor rotation.

- However, the energy coupling mechanism of the flagellar motor remains unknown.

DIRECTONAL OF FLAGELLAR MOTOR ROTATION

- E. coli and Salmonella cells sense temporal changes in chemical concentrations of attractants and repellents via transmembrane chemoreceptors localized near the cell pole .

- The binding of repellent to MCP induces auto-phosphorylation of CheA via the adopter protein CheW, and then CheA-P transfers a phosphate to the response regulator CheY.

- The binding of the phosphorylated form of CheY (CheY-P) to FliM and FliN induces the structural remodeling of the C ring responsible for the switching of direction of flagellar motor rotation from CCW to CW.

MECHANISM OTHER THAN FLAGELLAR ROTATION-

- Spirochetes show several type of movementsuch as flexing,spinning,free swimming and creeping.

- Within the cell envelop flagella like structure which are known as periplasmic flagella or axial fibrils.

- The axial fibrils are present in the space between inner and outer membrane of cell envelop.

Swarming -

- This motility occurs on moist surfaces and is a type of group behavior in which cells move in unison across the surface.

- Most bacteria that swarm have peritrichous flagella. Many also produce and secrete molecules that help them move across the substrate.

- When bacteria that swarm are cultured in the laboratory on appropriate solid media, they produce characteristic colony

Spirochete Motility -

- Many spirochetes, multiple flagella arise from each end of the Swarming Bacteria Often Produce Distinctive Patterns on a Solid Growth Medium.

- These bacterial cells swarmed out from the center of the plate and produced a branching pattern called dendrites.

- The periplasmic space and are covered by the outer membrane.

- They are called periplasmic flagella.

- They are thought to rotate like the external flagella of other bacteria, causing the cork screw-shaped outer membrane to rotate and move the cell

Twitching and Gliding Motility -

- Twitching and gliding motility occur when cells are on a solid surface.

- Both types of motility can involve type IV pili.

- Thus they are considered together.

- The gliding motility of Flavobacterium spp. and Mycoplasma

- Motility of some bacteria. Twitching motility is characteized by short, intermittent, jerky motions of up to several micrometers in length and is normally seen on very moist surfaces.

- The pili are thought to alternately extend and retract

to move bacteria during twitching motility.

CHEMOTAXIS-

- Chemotaxis is the directed motion of an organism toward environmental conditions it deems attractive and away from surroundings it finds repellent.

- Movement of flagellated bacteria such as Escherichia coli can be characterized as a sequence of smooth-swimming runs punctuated by intermittent tumbles.

- Tumbles last only a fraction of a second, which is sufficient to effectively randomize the direction of the next run.

- Runs tend to be variable in length extending from a fraction of a second to several minutes.